The invention relates to a highly reliable solid electrolyte capacitor and method of making, and more particularly to a solid electrolyte capacitor including an anode and cathode each composed of carbon and RbAg.sub.4 I.sub.5 and a solid electrolyte composed of RbAg.sub.4 I.sub.5, and also to a method of using the capacitor in a pseudocapacitance mode to increase energy storage without causing dendrite growth that would electrically short the anode of the capacitor to the cathode thereof.
In this and the following discussion, the term "anode" will be used for the negative electrode and the term "cathode" will be used for the positive electrode, even though this terminology is strictly correct only for the discharge process.
Certain low voltage solid-state "electrochemical cells", perhaps more accurately referred to as "supercapacitors", are known and in some ways are advantageous compared to conventional batteries with respect to shelf-life stability, leak-free properties, and miniaturization. Such solid-state cells generally utilize silver as anode material and carbon capable of functioning as an electron receptor material for the cathode material. The solid electrolyte serves as an ionic conductor for the ionic part of the current within the solid-state cell. U.S. Pat. Nos. 3,701,685, 3,503,810, 3,443,997, 3,476,605, 3,647,549, 3,663,299, and 3,476,606 are generally indicative of the state of the art for such solid-state cells.
As indicated in "A Solid-State Energy Storage Device", published in "The Proceedings of the Power Sources Conference", No. 24, 1970, page 20, and as indicated in "A Solid-State Electrochemical Capacitor" by J. E. Oxley, published as Abstract No. 175 in a source unknown to applicant, of unknown date, the feasibility of constructing such a capacitor (hereinafter referred to as a "supercapacitor"), based on the "double layer capacitance" inherent in an electrode/solid electrolyte interface has been recognized. The first reference mentioned above describes a supercapacitor, on page 20, as one of the type under discussion charged to a voltage below 0.50 volts wherein silver is plated on silver already present in the anode during charging and a layer of electrons is formed on the surface of carbon contained in the cathode during the charging operation.
The second reference mentioned above describes the same supercapacitor cell, being charged to levels in the range from 0.50 volts to 0.65 volts. The same structure in this case is referred to as a "pseudocapacitor" wherein more of the silver is plated onto the anode, and in the cathode a monolayer of iodine ions is postulated to form on the surface of carbon contained in the cathode material. This has the result of increasing the energy storage by a factor of approximately 5 over the energy storage in the same supercapacitor structure if no iodine is being plated onto the cathode. This increase is postulated to result from the decreased dielectric separation.
The electrochemical capacitor disclosed in the above references uses the highly conducting solid electrolyte RbAg.sub.4 I.sub.5. The anode of the basic cell system is composed of silver/RbAg.sub.4 I.sub.5 /carbon. The silver in the anode behaves as an electrode with very high capacitance, because the overvoltage necessary to plate silver on silver is very low.
At potentials between 0.50 volts and 0.66 volts, the RbAg.sub.4 I.sub.5 is oxidized to form a monolayer of iodine at the carbon/electrolyte interface in the electrolyte contained in the cathode.
At applied potentials below 0.50 volts, the charge is stored as electrons on the surface of the carbon of the cathode. When a potential slightly above this range is imposed on the solid-state cell, the current decays to near zero as the required activity of iodine is established at the carbon/electrolyte surface. Thus, when a voltage in the range of 0.50 volts to 0.65 volts is applied between the cathode and anode, its charging behavior changes and this is termed the "pseudocapacitance" region of the solid-state cell's capacitance characteristic. At potentials greater than 0.66 volts, the electrolyte is decomposed. When the cell is charged into the pseudocapacitance region, the total energy stored in the solid-state cell increases to about 5 times that stored in the double layer region.
Before describing the present invention, it will be helpful to better understand the details of the closest prior art. In the prior art structure shown in FIG. 1, capacitor 1 has an anode 2 composed of activated carbon, silver and RbAg.sub.4 I.sub.5. Anode 2 is connected by a conductor 3 to a negative voltage. Anode 2 abuts a solid dielectric 4 composed of pure RbAg.sub.4 I.sub.5. The opposite face of solid electrolyte 4 contacts a cathode 5 composed of activated carbon and RbAg.sub.4 I.sub.5. In prior art capacitor 1, the material of anode 2, solid electrolyte 4, and cathode 5 all include approximately one to two percent uniformly distributed LEXAN plastic material which serves as a binder for the particles of carbon and RbAg.sub.4 I.sub.5 of which capacitor 1 is composed. Capacitor 1 of FIG. 1 is referred to as a "polar" structure because the anode and cathode are separately composed as described above for a specific polarity.
A problem with use of a plastic binder such as LEXAN in capacitor 1 is that it appears to encourage growth of silver dendrites from the anode to the cathode. FIG. 1A illustrates growth of such silver dendrites, which cause capacitor failure by short circuiting the anode and the cathode together.
If a constant charging current is supplied into positive cathode conductor 6, the voltage across capacitor 1 has the characteristic shown in segment 30A of FIG. 2. The voltage rises as indicated by segment 30A up to 0.50 volts. In the past, a number of workers have charged capacitor 1 to a voltage greater than 0.50 volts, thus adding the pseudocapacitor function. Segment 30A designates what is referred to herein as the "double layer" operating region of capacitor 1 and segment 30B is referred to herein as the "pseudocapacitor" operating region.
The charging circuit of capacitor 1 includes silver ions (Ag.sup.+) flowing in the direction of arrow 7 from RbAg.sub.4 I.sub.5 electrolyte layer 4 to anode layer 2. The charging current also includes electrons (e.sup.-) flowing in the direction of arrow 8 from electrolyte 4 to cathode 5. A monolayer of such electrons are thought to become "plated" on activated carbon surface areas such as 15, possibly with a several angstrom gap maintained by molecular repulsive forces. The capacitance produced by the above mechanism is indicated by capacitor 19 in the schematic diagram of FIG. 3.
In FIG. 3, numeral IA designates a schematic equivalent diagram of the capacitance of the supercapacitor 1 shown in FIG. 1. Capacitor 19 represents the "double layer capacitance" and capacitor 20 designates the "pseudocapacitance". Approximately eighty percent of the energy storage capacity of capacitor 1 is in the "pseudocapacitance" range 30B of FIG. 2 when capacitor 1 is charged to a voltage of approximately 0.65 volts and twenty percent is in the "double layer" region 30A of FIG. 2 when capacitor 1 is charged to 0.50 volts. In the pseudocapacitance range, charge storage is thought to be due to accumulation of iodine ions on carbon surfaces of cathode 5.
The LEXAN binders mentioned above are thought to produce grains or growth paths in capacitor 1, particularly in RbAg.sub.4 I.sub.5 of electrolyte 4, which encourage growth of the above-mentioned silver dendrites. Such silver dendrites, illustrated by numeral 21 in FIG. 1A, may result in the primary failure mechanism of the prior art cell of FIG. 1.
There are numerous applications in which there is a need for a rechargeable cell that 1) can be completely discharged thousands of times, 2) is capable of operating reliably between -65 degrees Centigrade and +160 degrees Centigrade or higher, and 3) is highly reliable, with long lifetimes despite conditions of high temperatures and numerous repeated temperature cycles.